The present invention relates to medical devices for removing and/or breaking down occlusions or other blockages in blood vessels and more particularly to a catheter employing a heatable tip for removing and/or breaking down such occlusions or blockages so as to restore the normal flow of blood within the cardiovascular system.
Atherosclerosis is a major cause of heart disease in the United States. In atherosclerosis a plaque forms which, if allowed to accumulate, can occlude the artery completely. In the past, angiographic catheters have been used to clear a blocked vessel by, for example, scraping a vascular wall by using a needle attached to the distal end of a catheter, as in U.S. Pat. No. 4,465,072 to Taheri.
Balloons connected to catheters have been used in a procedure called angioplasty to compress the plaque to open the vessel. A similar approach is shown in U.S. Pat. No. 4,589,412 to Kensey in which a cutting tip at the distal end of a catheter is rotated by the application of fluid pressure to cut away the plaque on the vessel walls. This catheter also includes a balloon which is disposed proximal of the cutting tip and is inflatable to protect against possible perforation of the artery. Two significant drawbacks of the these types of devices reside in the danger of passing a sharp object, such as a scraping needle or cutting tip, through a blood vessel, and, repeated inflation and deflation of a balloon in the blood vessel results in a residual state of stress and/or fatigue being imposed upon the walls of the vessel.
It is well known that fatiguing or otherwise traumatizing the walls of a blood vessel has an adverse effect on the bioelectrical properties of said vessel. Such an effect can have drastic long term consequences when it is realized that blood in its normal pH is negatively charged, and that there is approximately a one to five millivolt potential difference across the wall of a blood vessel. Should that potential difference decrease for any reason, such as where the vessel is traumatized and thrombosis is induced, blockage may build up on the vessel wall. For instance, as reported by Schwartz in a 1959 article "Prevention and Production of Thrombosis by Alterations in Electric Environment", Surgery, Gynecology and Obstetrics, page 533, and page 536, May 1959, the imposition of five millivolts of positive potential upon a segment of superficial femoral vein which has been occluded proximally results in thrombosis after one hour.
Laser beam angioplasty is becoming well established as having significant potential in the treatment of totally or severely obstructed blood vessels. In laser beam angioplasty a laser probe is first used to vaporize a minute pathway through the plaque. A small guide wire is slid through that pathway and a balloon catheter is passed over the guide wire to dilate the vessel in the conventional manner. U.S Pat. No. 4,207,874 to Choy describes such a laser tunnelling device which includes a fiber optic bundle for actually viewing the occlusion, a laser for vaporizing a tunnel through the occlusion, an injection port for radiopaque material so that the tip of the catheter may be accurately located, and a suction port for removing the vaporized material. Other such laser angioplasty devices are known. See for example, U.S. Pat. No. 4,627,436 to Leckrone.
Laser beam angioplasty systems are not without disadvantages. For one, the laser beam creates a very small pathway through the plaque. Moreover, these systems are not equally effective against all types of blockages due to the color absorptive specificity of various components of the atheroma and thrombi. In addition, there exists the possibility of perforation of the vessel by a laser beam focused too long on the vessel wall or positioned at an angle too acutely with respect to the vessel wall. Also many laser beam angioplasty systems lack the capability of being used with a guidewire to guide the laser cathether accurately to the occlusion.
According to the Sept. 3, 1986 issue of the Medical Tribune, Trimedyne, Inc. of Santa Ana, Calif. has a laser probe for coronary angioplasty having a metal alloy tip at the end of the probe which is heated by a laser beam to 400.degree. C. to vaporize obstructions in blood vessels. Apparently this probe is led to the obstruction over a guide wire placed in the blood vessel.
Radio frequency energy has previously been used in medical surgery. Such devices both cut and cauterize as illustrated by U.S. Pat. No. 3,089,496 to Degelman, for example. However, it is believed that such devices in the past have been ill-suited for the vaporization and removal of plaque in blood vessels because of the possibility of perforation of the vessel wall, and because of the size and complexity of such devices.